Gas tube circuit



`Oat. l5, 1946. H. J. woLL GAS TUBE CIRCUIT Filed March 31, `1944 attorney Patented. Oct. 15, 1946 UNITED STATES PATENT OFFICE.,I

GAS TUBE CIRCUIT Harry J. Woll, Indianapolis, Ind., assgnor to Radio Corporation 'of America, a corporation of Delaware Application March 31, 1944, serial No. 528,865

6 claims. (o1. S15- 152) This inventionV relates to circuits for gaseous discharge tubes and, more particularly, to time delay circuits for controlling the operation .of such tubes.

In many applications, time delay combinations are employed to delay the ring of a gas tube or to delay the extinguishment of space current in a gas tube alreadyfired. Where alternating currents are applied to the electrodes of the gas tube, space current therein is extinguished on each negative half cycle ofthe anode voltage; to delay extinguishment of currentin a tube so operated, it is `necessary to delay ring thereof during the subsequent positiveghalf `cycle or cycles. `It is frequently important to control the duration of that delay with accuracy andto make it independent of extraneous factors tending to vary theduration ofthe period of delay.l

By way of example; reference may be made to the control system shown and vdescribedin the co-pending application of J. H. Reynolds, Serial No. 526,524, liled March 15, 1944, entitled Photoelectric relay and assigned to the same assignee as thevinstant application;V Thepresent inven-y tion, While constituting an improvement on the circuit Aof the control system described in that application, is not limited in scope to that circuit butj may be employed whenever it is desired to delay the operation of a gas `tube with accuracy.

In the control system ofthe application referred to, a beam Aof light is directed to a phototube and current from the photo-tube fires a gas tube, which in turn energizes a relay; the latter controlsthe operation c f a conveyor along which objects are moved in spaced relation. These objects intermittently interrupt the light passingv to the photo-tube'. This would ordinarily extinguish current in the gas tube, except that a time delay circuit, connected between the grid and the cathode' of the gastube, delays such extinguishment for a period at least equal to the time taken by an object totraverse the beam. When, however, objects become jammed on the conveyor and the light passing to the photo-tube is interrupted for more than a predetermined period, current in the gas tube is extinguished, the relay opens and the operation of the conveyor ceases.

Among the factors which tend to cause variation in'the period of delay in this and similar applications are the following:

l; Variation in supply voltagef--This will affect the `lament and anode voltages of the gas tube and, therefore, its sensitivity; it will also alect the anode voltage of thephoto-tube and, therefore, the voltage impressed by that tube on CTL thecapacitor of the time delay circuit. If there are other tubes in the circuit, their electrode voltages will also be affected by variation in supply voltage and this, in turn, will `affect the delay period. I

2. Variation in the intensity of light-Varia-l tions in supplyA voltage will affect the intensity of light and, therefore, the delay period; but light variations maybe due to other causes. 3Q Variations in the characteristics of tubesq-j Since noY two `tubes are c onstrucud precisely alike, there 'may on replacement of tubes be 'a differencein the characteristics of the two tubes due to suchdifferencein construction, and, hence, a further Variation in delay. 'M Qreover, the crit: ical grid voltage` which will cause the tubeto re may vary in any one tube, depending on itsfmagej, by asmuch as4 one volt. Both these factors `will affect the period of delay; Y A `It is, therefore, an object gof the'invention to provide. animproved circuit for a gas tube, including a time delay combination wherein the period of delay will be substantially constant and independent of factorstending to vary it. g

A further object of the invention is to provide an improved circuit 4of the character described, wherein the period 'ci ldelay'willbe independent of variations in supply voltage. l l

A still further object is the provision of an mproved circuit for a lgas tube, which includesV a light-responsive element and a time delay 'com-, binatiori, and in which the delay will be inde--` pendent of variations inthe intensity of light passing to that element.

Another object is to provide atime delay cir" @uit for a gas' tube which will have a delay in-Q dependent `of the characteristics tube employed therein. According to the invention', these opbJ of the` particular` ects are l achieved by providing in thegrid circuitfof the gas tube, inaddition to the time delay combination, a resistor and capacitor in parallel therewith, so connected and of such Values as toproduce a variation in the bias applied to the grid,

which counterbalances the'effects caused by vari-' ations of the character described. l l

' The' invention may be better understood from a 'consideration vof 'the 4v,following description of a" preferred embodiment' thereof, when read in ycon junction with the accompanying' drawing, the single figure of which is a schematic diagram of a circuit embodying the principles of the invention which may be used in a photo-electric controlsystem.

Referring to the drawing, the circuit includes and capacitors in 3 a photo-tube I and two gas tubes I2 and I4. The photo-tube, which may be an RCA type 868, has an anode I 6 and a cathode I8. Each gas tube has an anode 20, 22, a, cathode 24, 26 and a control grid 28, 38, respectively. The two gas tubes may be an RCA type 2051.

All three tubes are connected in cascade, the cathode I8 of the photo-tube being connected to the control grid 28 by a conductor 14, andthe cathode 24 of the rst or buffer gas tube to the control grid 38 by a resistor 16. The rst gas tube isolates or buiiers the second or control` ling gas tube from the photo-tube and makes it independent of variations in theintensity of light passing to the photo-tube. y

Alternating potentials for the electrodes of the three tubes are obtained from any appropriate source of alternating voltage (not shown) through atransformer 36. The voltage supply may have a frequency of 60 cycles per second and an output which varies between 105 volts and 125 volts. The transformer 3B has a secondary winding 38, one end 42 of which is connected by a conductor 40 togthe anodes 20 and 22 of the two gas tubes. The anode potentials of the gas tubes of the preferred embodiment should not exceed 350 volts. The anode I6 of the photo-tube, on the other hand, requires only 90 volts and for this purpose a voltage divider, consisting of resistors 62. and 64, is connected across the secondary winding 38; a resistor 90 and a capacitor |06 are alsoconnected across this winding for purposes hereinafter explained. The anode of the photo-tube is connected by a conductor 66 toa terminal`68 between the two resistors 62 and 84. By way of example, the values of all the resistors the circuit are marked on the drawing.

The gas tubes I2 and I4 have filaments 60 and 48, respectively. Voltage for the iilament 48 is derived from a portion of the secondary winding 38 between a, tap 44 thereon and an end 52. Current is applied to the filament by conductors 46 and 50. The lament circuit so formed also provides voltage for an exciter lamp 88. Voltage for the iilament 6|! is derived from an auxiliary secondary winding 54 of the transformer 36. Current is applied to the lament through conductors 84 and 88. l f

The cathode I8 of thephoto-tubeis connected to one end 58 of the auxiliary secondary Winding 54 by the conductor 86 and a resistor 1D; in parallelwith this resistor is a capacitor '|2. The terminals of the resistor and capacitor adjacent the photo-tube are connected to the conductor "I4 (and, therefore, to the control grid 28) at the junction 88 of the conductor 14 and the cathode I8. By this means. an alternating bias derived from the voltage on the lament 6D is applied to the control grid 28. The bias prevents gas tube I2 from firing in the absence of a ring potential derived from the photo-tube. In a similar fashion, anl alternating bias is derived from the voltage onthe filament 48 and applied to the control grid 38 through a capacitor 92, connected to the conductor U (in the lament circuit of tube I4) and the resistor 16, which connects the cathode 24 tothe control grid 38. The bias prevents the tubev I4 from firing unless a positive potential is derived from the cathode circuit of the tube I2.

In parallel with the capacitor 92, in the grid circuit. of the tube I4, is the resistor 98. The resistor and capacitor together constitute a. time delay'combination, their values being Selected. to

provide the delay desired. In this embodiment, the product of resistance and capacitance provides a time constant of .5 second, but other Values may be chosen as desired. The resistor 'I6 also has some eiect on the time delay. A resistor 94 connects the capacitor 92 to the cathode 24 of the tube l2. The resistor 94 must be large enough to limit current iiowing through tube I2 to a safe Value, and yet small enough to charge the capacitor 92 quickly when tube I2 lires, the charging time being determined substantially by the time constant of the combination of resistor 84 and capacitor 92. In the preferred embodiment, a value of 680 ohms was found satisfactory for resistor 94.

A capacitor 86 is also contained in the grid circuit of tube I4, being connected at one terminal to the end of the resistor 'I6 adjacent the grid 38, and at the other terminal to the filament 48. This capacitor and the resistor I8 determine the magnitude of the bias applied to grid 30 and thereby automatically compensate for the eiTects on the tube I4 caused by variations in the supply voltage, as hereinafter explained.

A relay 98, in operative connection with a switch |88, is connected in the anode circuit of the tube I4 between the anode 22 and the end 42 of the secondary winding 38. In order to damp out sharp, transient impulses caused by voltage which may be built up in the energizing coil of the relay when current in the tube I4 is extinguished, a resistor |02 and capacitor |64 are connected between the .relay and the cathode 26. Irhe capacitor |86 in the anode circuit of the photo-tube prevents the charge built upon the capacitor 92 from reaching the anode of the photo-tube.

The operation of the circuit may be explained as follows: When Vthe photo-tube is not excited, the two gas tubes I2 and |4 are prevented from ring by the alternating bias applied to their grids and derived from their respective lament circuits. The respective instantaneous polarities of the ends of the two sections of the secondary winding 38 and auxiliary secondary winding 54 are shown on the drawing. When the anodes of the two gas tubes are most positive, their grids are most negative, and vise versa. When each anode is about to commence on its positive half cycle, each grid commences on its negative half cycle. In the case of tube I2, the capacitor 'I2 shifts the phase of the bias slightly. Moreover, the combination of the resistor 'I8 and capacitor 'l2 acts as a slight filter of the pulsating voltage appearing across the resistor when grid current is drawn on the positive swing of the bias.

Both of these efrccts make the grid 28 somewhat more negative when the anode potential is zero and is about lto commence on its positive half cycle. This contributes to prompt and accurate extinguishment of the tube I2, which is important if the period of delay is to be accurately controlled. The combination of the resistor 'I6 and the capacitor 96 in the grid circuit ofthe tube 4 has a similar effect in helping to extinguish that tube. The action of the resistor |82A 'and capacitor |84, tending in the same direction, has already been explained.

When light from the lamp 88 is permitted to pass to the cathode I8 of the photo-tube, current passes from that cathode to the control grid 28 of the buffer tube l2. The voltage built up on the grid overcomes its bias and lres the tube. Space current then flows in the cathode circuit and passes along'at least two paths; one path is -through the ,resistor 94 and, therefore, builds up other path is through the resistor 16 to the control grid 3U, whose bias isA overcome in the same fashion as in the case of the other gas t'ube.` The controlling tube I4 is thus fired, the relay 98 energized and the switch closed.

When the passage of light to the photo-tube is interrupted, the positive voltage derived therefrom is removed from'the control grid 28 and the space current of the tube I2 is extinguished. The capacitor 92` is then discharged mainly throughrthe resistors 90, 94 Yand 16, Since the resistor 94 is small by comparison with thetwo resistors 9i] and 16, it has very little effect in this connection; The desired time delay is, in general, dependent on the time constant of the resistor-capacitor combination Sil-92. When the potential of the grid 30 falls sufficiently, the controlling tube I4 is extinguished and switch m0 is opened. Y i

For the consistent extinguishment of space current in the tube I4, `the grid 30 must not be permitted to rise to a value which will iire the tube on the positive half cycle of the anode voltage. That value will, however, vary inversely with the anode voltage which, in turn, is a function of the supply voltage. The :voltage onthe grid 3i! (after extinguishment of space current in the tube i2) is the algebraic sum vof the charge on, the capacitor 92 and the bias derived from the filament circuit of the tube I4. The function, therefore, of the resistor-capacitor combination 16, 9B is to so vary the grid voltage as to offset changes caused by variations in the supply voltage. If the supply voltage increases, for example, the combination of the resistor 1G and the capacitor 96 increases the bias and alters its phase, and, therefore, the tube I4 is not affected by the variation inv supply voltage. A decrease in supply voltage has an opposite effect. The delay is, therefore, constant and independent of variations tending to affect it,

At 60 cycles per second (the frequency of the current flowing in the filament circuit of the controlling tube I4) ,the impedance of the capacitor S2 and the resistor 94 may be regarded as negligible, and the resistor 16 is, therefore, effectively in parallel with the capacitor 96.

The resistor 1B must be large enough to prevent the capacitor 92 from discharging through it too rapidly. Since grid current passes through the resistor, a large resistance would emphasize differences in characteristics between individual tubes. The greater the value of the capacitor 96, the greater is the bias on the grid 3D. In the embodiment described, it was found that with a value of 1 megohm for the resistor 16 and 2700 micro-microfarads for the capacitor 96, any change in anode and filament voltages and in the voltage across the capacitor 92, was balanced by a change in the bias applied to the grid 30. The delay in extingiushment of the tube I4 was maintained constant within limits of f 5 per cent, notwithstanding variations in supply voltage, light intensity and tube characteristics.

Since the discharge curve of the capacitor 92 is an exponential one, it was found desirable to maintain the charge on the capacitor high, so that any variation in the firing potential of the grid 30, caused minimum variation in delay.

There has thus been described a circuit for a gas tube, including a time delay circuit in which a resistor and capacitor in parallel therewith in the grid circuit of the gas tube create changes in the bias of the grid, which counterbalance changes due to variations in supply voltage, light intensity and tube characteristics. The time delay of the time delay combinationis, therefore, enabled to remain substantially constant.

I claim as my4 invention: i

`1. In combination, a gas tube having a control grid, a source of firing potential for said'grid, a resistor connecting said source to said grid, terminal means for connectingia sourcelof alternating electrical energy to said tube to apply alternating potentials tothe anode and filament of said tube and an alternating bias to said grid to prevent conduction of said tube, a time delay circuit connected to said grid to delay extinguishment of space current in said tube during positive half cycles of the alternating potential applied to 'said anode on interruption of said source of ring potential, and a capacitor'effectively in parallel with said resistor for varying said-bias as a function of any variation in the voltage output of said source of alternating electrical energy, whereby to make said delay independent of said variation.

2. In combination, a gas tube having a control grid, a source of-firing potential for said gridincluding an element adapted to have an alternating potential applied thereto, a resistor connecting said source to said grid, terminal means for connecting asource of alternating electricalenergy to said ,element and `said tube to applyv alternating potentials to said element and to the anode and filament of said tube, and an alternating bias to said grid to prevent conduction of said tube, a time delay circuit connected to said grid to delay extinguishment of space current in said tube during positive half cycles of the alternating potential applied to said anode on interruption of said source of firing potential, and a capacitor effectively in parallel therewith for varying said bias as a function of any variation in the voltage output of said source of alternating electrical energy, whereby to make 'said delay independent of said variation.

3. In combination, a controlling gas tube having a control grid, a source of firing potential for Said grid including a light-responsive element, a buffer gas tube connected between said element and said grid, a resistor connecting said buifer tube to said grid, terminal means for connecting a source of alternating energy to said tubes to apply alternating potential to the anodes and filaments of 'said tubes, respectively, and an a1- ternating bias to said grid tc prevent conduction of said controlling tube, a time delay circuit connected to said grid to delay extinguishment of space current in said controlling tube during positive half cycles of the alternating potential applied to the anode of said controlling tube on interruption of Said source of firing potential, and a capacitor effectively in parallel with said resistor for varying said bias as a function of any variation in the voltage output of said source of alternating electrical energy, whereby to make said delay independent of said variation and of any variation in the intensity of light passing to said element.

4. In combination, a light-responsive device having an anode and a cathode, a source of light passing to said cathode, a controlling gas tube, a

buffer gas tube, each of said tubes having a control grid, terminal means for connecting a source of alternating electrical energy to said device and said tubes to. apply alternating potentials to the anode. of said device and the respective anodes and filaments of said tubes, and alternating biases tothe grids of said tubes to prevent conduction of said tubes, said device, said controlling tube and Said buier tube being connected successively in cascade with said device whereby to re said tubes on the passage of light to the cathode of said device, a time delay circuit connectedY to the grid of said controlling tube to delay extinguishment of space current in said controlling tube during positive half cycles of the alterna+- ing potential applied to the anode thereof on interruption of light passing to the cathode of said device, and a resistor connecting the cathode ci said buffer tube to the grid of said controlling tube and a capacitor eiectively in parallel with said resistor for varying the bias of 'said controlling tube as a function of any variation in the voltage output of said source of alternating electrical energy, ywhereby to make said delay independent of said variation and independent of any variation in the intensity of the light passing to the cathode of said device.

5. In combination, a gas tube having a control grid, a source of iiring potential for said grid, a firstl resistor connecting said source to 'said' grid, terminal means for connecting a source of alternating electrical energy to said tube to apply a1- ternating potentials to the anode and ilarnent of said tube and analternating bias to said grid to preventA conduction of said tube, a time delay circuit connected to said grid to delay extinguishment ofspace currentin said tube during positive half cycles of the alternating potential applied to said anode on interruption of said source of ring potential, said circuit including a sec- 0nd resistor and a rst capacitor in parallel therewith, a third resistor of low resistance connecting said source of ring potential to Said rlrst capacitor, whereby on cessation of said interruption said first capacitor will charge quickly by comparison With said delay, and a second capacitor effectively in parallel with said rst resistor for varying said bias as a function of any variation in the voltage output of said source of alternating electrical energy, whereby to make said delay independent of said variation.

6. In a gas tube which has (a) at least one control electrode in addition to a cathode and an anode, (b) a circuit connecting said control electrode to said cathode and including a time delay combination and (c) connections to a source of alternating voltage for applying an alternating potential to said anode and an alternating bias to said control electrode to prevent conduction of said tube, the improvement which consists in including in said circuit in addition to said time delay combination a resistor and a capacitor effectively connected in parallel with said resistor whereby to vary said bias as a function of any variation in the voltage of said source and thereby to make the delay created by said time delay combination independent of the variation in said source.

HARRY J. WOLL. 

